Continued studies on the growth of megakaryocytes revealed a very fine structure of the demarcation membrane system of the cell, a phenomenon which has not yet been observed in cultures. Motion picture studies on the growth of megkaryocytes showed various stages of maturation of the cells, including endomitosis, marked motility, and formation of the demarcation membrane system, followed by extrusion of numerous cytoplasmic projections suggesting the phenomenon of platelet liberation.